RS Watanabe – The Magic of Unsprung Weight

I finally got my Watanabes on the car. And oh my god! Not only does it look amazing but the lightness of the rims made a bigger difference than expected (especially when comparing to the heavy mazdaspeeds I had on for 4 years). The fitment/offset was far better than expected. In fact, I had some 20mm hubcentric spacers that i was going to put on but I guess they’re up for sale now. Right after I had them on this morning, I drove up to the local mountain pass (Black Mountain) for some spirited driving. It. Was. Awesome.

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However, on the downhill, I did notice a bit of scrubbing under hard braking and cornering. May require some guard rolling in the future, but it’s most likely due to the old stock suspension. Chances are that the shocks are pretty much dead which is perfect timing for the next part up on the list. 😀 Especially given the car looks like a 4WD, begging for some lowering.

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Everyone knows that the lighter your car the better the acceleration, braking and handling. But there is important distinction in weight when it comes to weight reduction. Specifically, it’s the concept of sprung and unsprung weight. Sprung weight refers to the parts of a car that are supported by the suspension/wishbone links. In essences, they hold the vehicle’s frame, engine, body, driver and everything that are above the wheels. Of course, these parts make up most of the weight of the car. On the other-hand, unsprung weight refers to parts such as the wheels, tires, and brake assemblies.

The diagram below demonstrates why unsprung weight is so important:

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The effect of reducing unsprung weight is that the effect of weight reduction is far more effective in improving acceleration, braking, and even ride quality! In the example above, if the unsprung weight is 50lbs, a 2G bump would result in a vertical force of 100lbs. On the other-hand, if the unsprung weight is 30lbs, the same bump would only result in a vertical force of 60lbs. The increase in unsprung weight will reduce the grip of the car, because the weight of the car is what keeps the tyre planted, and pushing a car up into the air with that much force will inevitably reduce the weight on the tyre, and hence grip. Essentially, the more upward force, the more difficult it is the keep the tires planted on the road. This is especially true for lighter weight cars like the MX-5. In the same light, ride quality is reduced as the same springs and dampers has to deal with a stronger upward force. As for acceleration, the reduction in unsprung weight essentially means that there is less mass for the engine/diff to turn. As a result, less force is required to turn the wheels resulting in faster acceleration.

Note: The performance difference at driving speeds are not massive, but you can definitely feel it on your ass 😛

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